Fyn kinase, a cytoplasmic Src family tyrosine kinase, plays an important role in many physiologic processes and is ubiquitous in human cells. Fyn kinase is primarily found on the cytoplasmic face of the plasma membrane where it phosphorylates tyrosine residues of a number of different enzymes involved in the signaling pathways associated with various cell-surface receptors. Fyn kinase is involved in cellular development, apoptosis, and homeostatic regulation, as well as in the development of many different kinds of pathologies. Fyn kinase represents an attractive target for drug therapies designed to control cellular metabolism for management of diabetes, pre-diabetes and general weight loss applications, as an anti-cancer therapeutic and as potential target for the control or prevention of Multiple Sclerosis, Alzheimer's and Parkinson's disease.
Fyn knockout mice exhibit increased lipid utilization and increased energy expenditure. Wild-type mice treated with the highly selective Src family kinase inhibitor SU6656, exhibit similar increases in lipid utilization and energy utilization, whereas the Fyn knockout strains show no increase in either parameter upon treatment with SU6656 (WO 2011/119199). Both the Fyn knockout and SU6656 treated mice have significantly reduced fat mass relative to untreated or genetically matched Fyn-competent control mice. Treatment with SU6656 appears to reduce adiposity and promote weight loss, likely through a Fyn-kinase-dependent mechanism.
Fyn kinases ability to regulate fatty acid oxidation and increase energy expenditure is thought to result from its affect on sequestration of LKB1 to the nucleus of skeletal muscle cells. Fyn kinase phosphorylates LKB1 present in the cytoplasm of such cells and the phosphorylated LKB1 is preferentially localized to the nucleus where it has little or no access to AMPK, which is found almost exclusively in the cytoplasm. Thus, LKB1 phosphorylated by Fyn kinase cannot activate AMPK (AMP-dependent protein kinase). Activated AMPK serves as a cellular energy sensor directly regulated by alterations in the intracellular AMP/ATP ratio that occurs during prolonged fasting and re-feeding. At high AMP/ATP ratios activation of AMPK results in phosphorylation and inhibition of Acetyl CoA Carboxylase, thereby increasing fatty acid oxidation and decreasing fatty acid biosynthesis. Specific inhibition of Fyn kinase allows LKB1 to constitutively activate AMPK. In this model inhibition of Fyn kinase results in increased cytoplasmic localization of LKB1, resulting in increased levels of activated AMPK.
AMPK also regulates glucose metabolism in liver. Hepatic glucose production that is not regulated by insulin is known to play an important role in development of Type 2 diabetes. The asymptomatic insulin resistance phase of Type 2 diabetes is generally followed by defects in insulin secretion that result in severe hyperglycemia if left untreated. Chronic high glucose levels have been shown to result in the death of pancreatic n-cells and loss of insulin production altogether. Activated AMPK reduces insulin secretion and is thought to have a pro-apoptotic effect on pancreatic β-cells.
Like many Src family kinases, Fyn plays a role in regulating several physiological processes including cellular growth, proliferation, morphogenesis and motility. Fyn has also been recognized as a potential oncogene and has been shown to be capable of inducing the fully tumorigenic phenotype. Fyn interacts with a number of cancer related pathways. Fyn is a mediator of growth-factor induced anti-apoptotic activity of Akt/PKB, and regulates Rac and Rho GTPases and activates the ERK/MAPK pathways. Fyn also plays a key role in the regulation of matrix formation and degradation, which is important for matrix remodeling and cellular adherence.
In addition to metabolic and oncogenic regulation, Fyn is recognized as playing a key role in T-cell development as well as dendritic maturation of cells within the central nervous system. Further, inhibition of Fyn kinase is viewed as an attractive therapeutic target for treatment of Multiple Sclerosis. Alzheimer's and Parkinson's disease. It is the object of this disclosure to describe pharmaceutically effective inhibitors of Fyn kinase with enhanced specificity for therapeutic treatment of one or more of the pathologies in which Fyn kinase plays a role.
Structurally, Fyn kinase is relatively well characterized despite the lack of a complete high-resolution structure. The enzyme comprises 537 amino acids in a single polypeptide chain organized into 4 domains, with a molecular weight of 59 kilodaltons. The N-terminal membrane anchoring domain (SH4) possesses myristylated or palmitylated residues essential for proper enzyme trafficking and localization to the cytoplasmic membrane, immediately adjacent to this are the SH3 and SH2 domains, which play critical roles in the interaction of Fyn kinase with its protein targets, followed by a flexible linker connecting the C-terminal tyrosine kinase domain (SH1). Although a complete high-resolution structure of the entire Fyn kinase protein is not available, such structures are available for each of the SH2, 3 and 4 domains, individually.
The SH2 domain of Fyn kinase binds phosphotyrosine-containing sequences and functions as a target recognition domain. The SH3 domain promiscuously complexes with polyproline peptides and plays a role in mediating protein-protein interactions between Fyn kinase and other polypeptides. Interaction between the SH3 and SH2 domains enhances the specificity of ligand binding and regulates the activity of the kinase (SH4) domain.
The well conserved SH4 domain contains two potential inhibitor binding sites, the first, sensitive to non-specific competitive inhibitors of ATP such as staurosporine, represents the actual ATP binding site of the kinase domain, whereas the second site, immediately adjacent to the ATP binding pocket, is oriented towards the SH2 domain. This second site may be occupied in such a way that ATP binding at the first site is not directly blocked. However, free access to the second site is blocked by the presence of ATP at the first site, and hence, inhibitors targeted to the second site display pseudo-competitive kinetics with respect to the availability of ATP. Although the ATP binding site is highly conserved among Src kinases, the amino acids comprising the hydrophobic pocket typical of the second site are less conserved and may represent the best target for Fyn-specific inhibitors. Regardless, the SH1 domain is essential for the activity of Fyn kinase on any of its protein targets, and has been targeted for the development of potent Fyn kinase inhibitors.
Existing Fyn kinase inhibitors all suffer from a lack of specificity and although many Fyn kinase inhibitors have been isolated, most significantly cross-react with other Src family enzymes. Some of the best known Fyn kinase inhibitors include: phenolic compounds such as rosmarinic acid, (−)-epigallocatechin gallate and myricetin; the pyrazolol[3,4-d] pyrimidines PP1, PP2, and various derivatives thereof; as well as other fused pyrimidine compounds such as benzyl 21-methoxy-5,7,19-trioxa-2,13,24,26-tetraazapentacyclo [18.6.2.03,110.04,80.023,27]octacosa-1(26),3(11),4(8),9,20,22,24,27-octaene-13-carboxylate (Janssen Pharmaceuticals, U.S. Pat. No. 8,492,377) and CT5263 and CT5102 (CellTech). The structure of a few of these compounds and their associated IC50 (when known) and log P values are presented in Table 1.
TABLE 1Known Fyn kinase inhibitors.IC50logPFYN(con-CommonStructureIUPAC name(μM)sensus)name(2R)-3-(3,4-dihydroxy- phenyl)-2-{[(2E)-3-(3,4- dihydroxyphenyl)prop- 2-enoyl]oxy} propanoic acid1.3  3.00rosmarinic acid (2R,3R)-5,7-dihydroxy- 2-(3,4,5-trihydroxy- phenyl)-3,4-dihydro-2H- 1-benzopyran-3-yl 3,4,5unknown3.08(−)-epigallo- catechin gallate 5,7-dihydroxy-2-(3,4,5- trihydroxyphenyl)-4H- chromen-4-oneunknown0.65myricetin 1-tert-butyl-3-(4- methylphenyl)-1H- pyrazolo[3,4-d] pyrimidin-4-amine0.006 3.23PP1 (Pfizer) 1-tert-butyl-3-(4- chlorophenyl)-1H- pyrazolo[3,4-d] pyrimidin-4-amine0.006 3.32PP2 (Pfizer) N-(4-{4-amino-1- [(1r,4r)-4-(4-methyl- piperazin-1-yl)cyclo- hexyl]-1H-pyrazolo [3,4-d]pyrimidin-3- yl}-2-methoxy- phenyl)-1-methyl-1H- indole-2-carboxamide0.330 3.81A-420983 (Abbott) benzyl- 21-methoxy-5,7,19- trioxa-2,13,24,26- tetraazapenta- cyclo[18.6.2.03, 11.04,8.023,27]octacosa- 1(26),3(11),4(8),9, 20,22,24,27-octaene- 13-carboxylate0.006695.13Compound B50 (U.S. Pat. No. 8,492,377) (Janssen Pharma- ceuticals) 9-methoxy-N-{3- methoxy-4-[3- (pyrrolidin-1-yl)propyl] phenyl}-6,6-dimethyl- 5H,6H-benzo[h] quinazolin-2-amine0.008 6.12CT5263 (CellTech) N-{4-[2-(dimethyl- amino)ethoxy]- 3,5-dimethyl phenyl}-9-methoxy- 5H,6H-benzo[h] quinazolin-2-amine0.008 5.49CT5102 (CellTech)